Infection of dogs with Dirofilaria repens has been diag-nosed in many European countries and is spreading. Itis also currently considered one of the most important,

emerging zoonoses. These guidelines, developed by theEuropean Society for Dirofilaria and Angiostrongylus,are based on the latest information and include up-to -date recommendations for the prevention, diagnosis,and clinical management of subcutaneous dirofilariosis.

Life cycle of Dirofilaria repens

Dogs and cats with subcutaneous dirofilariosis harboradult parasites (females approximately 15 cms, males 7cm) in subcutaneous tissue. Microfilariae are released,enter the bloodstream and are taken up by mosquitoes.Approximately 15 days later, larvae become infective L3that are inoculated into a new host. Following severalmonths of tissue migration, parasites complete their de-velopment and begin to release L1.

European Prevalence

The map below shows the European distribution of D.repens in dogs not receiving prophylaxis. Prevalence da-ta for cats is lacking. The movement of infected hosts,the presence of competent mosquito vectors and climate

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GUIDELINES FOR CLINICAL MANAGEMENTOF SUBCUTANEOUS DIROFILARIOSIS

IN DOGS AND CATSPrepared for and approved by the Executive Board of ESDA

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changes that allow the development and survival ofmosquitoes for longer periods of the year all contributeto the spread of infection and disease. This is why it isso important to administer preventives during the trans-mission season. In some areas of Europe, this meansyear-round treatment (see section “Prevention”).

Clinical signs of subcutaneousdirofilariosis

The majority of D. repens-infected animals remainasymptomatic. When present, the primary clinical signis the presence of one or more skin nodules. They can belocated in different anatomical sites and can measurefrom 0.5-3 cm. Sporadic reports of erythema, papules,alopecia and pruritus have also been described in dogswith natural D. repens infection, while one case of sub-cutaneous dirofilariosis mimicking a fibrosarcoma hasbeen described in a cat.It has been reported that nodules may show swelling af-ter being handled (this should not be confused with theDarrier’s sign typical of mast cell tumors). Cytology by fine needle aspiration of nodules presents amixed inflammatory infiltrate with or without a signifi-cant eosinophilic population. In many cases microfilari-ae, fragments of uteri from female nematodes and de-veloping embryos can also be observed.Ultrasound of nodules may show the typical double lin-ear parallel hyperechoic structures indicative of filarialnematodes (Fig. 3).On histology, nodules are granulomatous to suppurativeand may contain cross-sections of filarial nematodesand/or microfilariae. The infiltrate is mainly composedof lymphocytes, macrophages, plasma cells, neutrophilsand eosinophils in different proportions.

Diagnosis

The hallmark of diagnosis for subcutaneous dirofilario-sis is the observation of microfilariae larvae in blood. Inasymptomatic infections, this is usually an accidentalfinding during routine blood smears or during screeningfor D. immitis. It is important to be able to distinguish

microfilariae of D. immitis from those of D. repens.Species of microfilariae in the blood, in the pellet fromKnott test or membrane from filtration test can be con-firmed by specific PCRs.

When one or more nodules are present, diagnosis can al-so be aided by cytology and ultrasound. Surgical re-moval and histology will confirm clinical suspicion.

How to perform a Knott test for the identification of D.repens microfilariae.• Mix 1.0 mL of EDTA venous blood with 9.0 mL of

2% formalin in a conical centrifuge tube (a 2% for-malin solution can be prepared diluting a standard4% or 10% formalin solution for histology with dis-tilled or tap water)

• Invert the tube gently 4 times to mix the solution• Centrifuge for 3 minutes at 1500 rpm• Pour off the supernatant and add 1-2 drops of 1%

methylene blue and mix• Place a drop of the sample on a glass slide and cover

with a coverslip• Examine the slide under the microscope at 10x to as-

sess the presence of mf, and at 40x to observe themorphological features

• For maximum sensitivity, the whole sample should beanalyzed.

For a step-by-step illustration of how to perform testsfor the presence and identification of microfilariae, con-sult the ESDA website www.esda.vet

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Figure 1 - Dirofilaria repens in Europe.

* DNA of the parasite found in mosquitos; ● sporadic cases.

Figure 2 - Surgical removal of an adult D. repens from a skinnodule, using the mini-invasive technique.

Figure 3 - Ultrasound of a skin nodule showing the typical dou-ble linear parallel hyperechoic structures.

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Treatment

There is currently one drug registered for the adulticidetreatment for D. repens. Monthly treatments with mox-idectin + imidacloprid spot-on (Advocate®) is licensedfor adulticide treatment, microfilaria reduction and pre-vention in dogs (see below).In case of subcutaneous nodules worms can be mini-mally invasive removed without any sedation with a 19G needle connected to a syringe creating vacuum insert-ed into the nodule (Fig. 1).Because of the zoonotic potential, microfilaraemic dogsshould be treated monthly for 12 months with drugs ac-tive against microfilariae.

Prevention

Prevention of D. repens is very important not only toavoid clinical diease in pets, but also to protect publichealth.D. repens infection can be prevented by the administra-tion of macrocyclic lactones that are able to eliminateinfective larvae up to 30 days old. Thus, the monthly ad-ministration will kill all the larvae that mosquitos haveinoculated in the previous 30 days. There are currentlydrugs that are registered for the prevention of D. repensinfection and D. immitis together.The use of topical synthetic pyrethroids (i.e. permetrin),applied monthly, has been reported as significantly de-creasing the risk of mosquitoes bites in dogs (so-called“anti-feeding” effect). The use of pyrethroids does notsubstitute the use of MLs, but it can be combined withmacrocyclic lactones to lessen the risk of infection in thecase of missed administration of an ML or in cases oflack of compliance of the owner (“double defense”).

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TABLE 1 - MACROCYCLIC LACTONES USED FOR THE PREVENTION OF SUBCUTANEOUS DIROFILARIOSIS

Drug Administration Dose Interval Efficacy against other ESDA parasites #

Ivermectin Oral > 6 mcg/kg Monthly D. immitis

Moxidectin Injection Sr 0.17 mg/kg Every 6-12 months D. immitis

Moxidectin Spot on >2.5 mg/kg Monthly D. immitis, A. vasorum

At the labelled dose, all these drugs are safe in dogs that are sensitive to MLs due to the presence of the so-called MultidrugResistant 1 mutation (for example, Collies, Australian shepherd).

# Prophylactic activity against D. immitis or A. vasorum. Some drugs alone or in combination are active against other endo or ectoparasites.

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